(a) Field of the Invention
The invention relates to a memory card control chip for accessing memory card, and particularly to a memory card control chip for accessing memory card using spread spectrum technology.
(b) Description of the Related Art
In recent years, many different types of flash memory cards are available. Compact Flash (CF), Secure Digital (SD), Multi Media Card (MMC), Memory Stick Pro (MSPRO), xD-picture (xD) and so forth are commonly seen. Due to the wide varieties of applications for flash memory cards, methods for accessing such memory cards are also developed for conveniences. Some manufacturers even equip the desktop or laptop PC with the functionality of accessing such new generation memory cards as the standard build-in accessories. Besides, due to the popularity of the peripherals for accessing memory cards, the requirements of the related specification also become harsher. The certification of electromagnetic interference (EMI) is a typical example.
FIG. 1 illustrates a traditional method for reducing the EMI intensity of a device for accessing memory cards. As shown in FIG. 1, the method utilizes external resistors. One additional resistor is placed in the signal path of the high frequency signal that generates EMI on the circuit board and the resistance of such resistor is generally between tens ohms to hundreds ohms. As an example of the traditional method, a resistor 106 is generally placed in the signal path that connects the SD clock pin of the memory card control chip 102 and the SD card socket 104. Similar approach is also used for other high frequency signals or other types of memory cards. But, as the EMI certification criteria become more stringent, the external resistor with higher resistance must be used for the memory card control chip in order to pass the EMI test. However, increasing the resistance not only costs more money but also reduces the driving ability of the clock signal. Even, incompatible problems may be appeared.